


Satan's Daughter

by Skycat3



Category: Ao no Exorcist | Blue Exorcist
Language: English
Status: In-Progress
Published: 2015-04-03
Updated: 2015-12-23
Packaged: 2018-03-21 03:37:30
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 11
Words: 4,289
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/3675939
Author URL: https://archiveofourown.org/users/Skycat3/pseuds/Skycat3
Summary: <blockquote class="userstuff">
      <p>Mild disclaimer- I don't actually speak Japanese, I'm trusting Google. Kasai means fire in Japanese ad apparently can be a name... I hope. If you speak Japanese and I'm terribly wrong.. I'm very sorry. Follows own storyline.</p>
    </blockquote>





	1. Meet Kasai

**Author's Note:**

> Mild disclaimer- I don't actually speak Japanese, I'm trusting Google. Kasai means fire in Japanese ad apparently can be a name... I hope. If you speak Japanese and I'm terribly wrong.. I'm very sorry. Follows own storyline.

She had been trying to make a Gehenna Gate and get to Assiah for a long time, but that took a lot of power. Power that she finally had. Just in time, too. He had to know his so called friends would't be his friends anymore if he told them. Or so she thought, from what she had viewed of human behavior.

"Is that a Gehenna Gate?" 

"Get back!" 

"Isn't Satan the only one who can make Gehenna gates?" Several voices spoke at once. Ow. Ground hurt. She had fallen face first from the Gehenna Gate. Someone ran over, then two more. She rolled over, opening her eyes. 

"Are you okay?" An anxious voice asked. A blonde girl with an infant green man on her shoulder was bending over her. Shiemi, if she recalled correctly. 

"I don't like being ignored!" Ah. Amiamon, King of Earth. Ugh. What a pain. She shakily got to her feet. 

"You need to rest!" Shiemi called after her. No, she didn't. Someone grabbed her shoulder. She turned around.

"Shiemi's right!" It was Rin. How odd. All these so called exorcists didn't seem to mind that she had just fallen out of a Gehenna Gate. She slapped his hand away, and Amiamon finally noticed her. 

He looked confused. "Kasai?" A smirk creeped across her face. She could beat Amiamon any day. Well, Kasai didn't know how she would do with most of her energy used to open the Gehenna Gate which had collapsed mere seconds after she fell through. An aura of blue flames surrounded Kasai, and she heard people gasp behind her. Rin and Yukio were confused, and the rest were angry or afraid, she couldn't tell.

"Hello, Amiamon." Kasai leapt, blue flames arcing in front of her. She burned away the earth, and the King of Earth that it surrounded. "How nice of you to stop by." Kasai pinned him to the floor with her foot. She turned to face the exorcists, the flames dying down. Their faces showed shock, fear, confusion. 

"What?" She heard Rin mutter. Yes. The Okumura brothers thought they were twins. They were mistaken. Then a strange feeling came over Kasai. What was it? She put a hand to her head, dizzy. Then she fell.

"Oh!" She heard Shiemi run over before she blacked out.


	2. True Cross Academy

Kasai struggled to consciousness. Something soft under her. She sat bolt upright. Kasai was in an unfamiliar place. She was on a bed, with blankets over her. She turned her head. Rin and Yukio were sitting next to the bed. Yukio was reading, but Rin was staring out the window. He heard her sit up. "Oh, you're up."

"Yep." She started to fully wake up. A smile played across her mouth. "Hey, brothers. How has your day been?" Rin looked confused, and Yukio looked annoyed. Hmm. He was probably mad because he was lied to. No one liked being lied to.

"Explain yourself." Short, terse. Fine then.

"Hey, she just collapsed, Yukio. Cut her some slack." Yukio's eyes narrowed.

"She has Satan's flames, and-"

"Yadda yadda going to shoot me blah blah I'd like to see you try." Her eyes narrowed, a smirk on her face. "If it's an explanation you want, then fine. If you hadn't noticed, brothers, I'm your sister!" Kasai smiled, closing her eyes. "One big happy incredibly dysfunctional family with a demon god as our dad!" Kasai's face straightened. "All joking aside, you two were raised in Assiah, and I was raised in Gehenna. I'm here to help Rin over there with his mission, because things will get ugly soon. Powerful demons are rising, and your best weapon can't even control his flames yet! Honestly."

Rin looked slightly offended. Yukio sighed. "So you'll teach Rin to use his flame?" Kasai nodded.

"And that other chick.. What was her name? Ah. Shura. She can teach him to use his sword." Yukio nodded.

"All right. You'd better wait here." Yukio got up to leave, and so did Rin. "You stay too, Rin."

"Aw, man! Why can't I go?" Yukio sighed.

"Just stay." With that, he left.

Rin turned back to Kasai, who was smiling again. "So, you're a child of Satan too?"

"Yes. Like you, I have the power of Satan's flames." Kasai swung herself out of bed and stretched. "I can't wait to start training!"


	3. Training

"Yukio said-"

"Yeah yeah. We aren't going anywhere!" Kasai stood up, yawning. "How bad is your control?"

Rin put a hand behind his head, his eyes downcast. "Well...." Kasai face palmed.

"I guess it helps to train with Satan." She looked around for something to burn. "Here. Watch." Fire spread from her palms. She made shapes with the fire, and it didn't burn anything she didn't want it to. It danced around the room, not to hot to touch. Rin's mouth hung open. Kasai smiled. "If you want to be able to do this, you have to train."

"Okay!" Rin opened his hands, and fire burst forth, uncontrolled. It burned Yukio's desk, and wildly burned. Kasai sighed.

"Rin! You have to start at the beginning! You're starting at the end." Kasai looked at the flaming desk. Rin tried to put out the fire, unsuccessfully. "Rin. Calm. Deep breaths. Get in control!" Rin nodded, closing his eyes and breathing slowly. The fire dimmed, finally going out. 

"So, where is the beginning?" Rin asked, falling back into his chair. Kasai was about to answer when Yukio opened the door. He looked at Rin and Kasai, and then at his burned desk. 

"What happened to my desk?" Yukio looked at Rin.

"Uh, uh, bye!" Rin raced out of the room. Yukio sighed.

"You'll be staying in the room next door. There isn't anyone else in the building, so you should be fine. Rin will probably make you lunch, dinner, and breakfast. Here is your schedule." Yukio handed Kasai her schedule showed Kasai her room. "Oh, and Lord Pheles wants me to give you this." He handed her a key on a chain. "He also issued you a uniform."

"What does it open?" Kasai took the key, putting it in her pocket.

"He wouldn't tell me." Yukio left. Kasai examined her new room. It was cosy, with a bed, a closet, and a desk with a chair. She didn't have anything to put on the desk or closet, but in one of the desk drawers was a pile of text books. In the closet was black pants, jacket, a white shirt, and a striped tie. Good. Not one of those ridiculously short skirts and frilly tie things. She looked at the key.

"I wonder what this opens."


	4. The Infinity Key

Kasai took the key off the chain after changing into her uniform. She looked at her schedule. Kasai had two hours before her first class started. She should study. Kasai quickly shoved the textbooks and schedule into a messenger bag and ran out the door of her room. She tried the key in her door first. It fit. She went to the next closed door. It was locked. Kasai put the key into the lock and opened the door.

"What the-" Suddenly she was in a bustling city square. Ah. It must be an Infinity Key, capable of linking any two places together via door. She quickly ducked back through the door and shut it. She could have fun with this. First things first though. Get to know the campus. Kasai made her way down to the first floor. Oh, and second thing- ask Samael, or Lord Pheles, why he tried to kill her brother. They were going to have words. 

"Who is she?"

"Is she new?"

"Why was she in the Okumura brother's dorm?" Humans. Not all of them exorcists. It must be a normal school also, then. Several of the aforementioned humans were staring. Kasai sighed. She didn't like most people as a general rule, and she didn't want to deal with a bunch of annoying teenagers. Even if she herself was a teenager. Well, she wasn't really a normal teenager, being the half-demon child of Satan and all. Anyways. The campus was huge. It was a town in and of itself. There were also lots and lots of humans. Everywhere. Kasai resigned herself to getting used to this. 

"Hey, uh, you!" Someone called. Kasai whipped her head around. The person wasn't talking to her. Or at least, wasn't now. Strange. Kasai couldn't make out who had spoken. Ugh. She checked the nearest clock. She had fifteen minutes to get to class and she was completely and utterly lost in a sea of annoying humans. Great. At least she had the Infinity Key. She quickly opened a door to the cram school and went through.

"Hey, Kasai!" Rin called her from a nearby table, patting the seat behind him. Shiemi sat next to him. Kasai started to move forward when a hand blocked her path. 

"Come with me. You too, Rin." Rin, surprised, got up from his seat and followed Yukio and Kasai out into the hall.

"Shura and I both agree that Kasai is your best chance of controlling your flames." Kasai smirked. "You're going to train with her." Rin nodded. Yukio gave Kasai directions to a room suitable for training. "Shura is going to keep an eye on you two." Kasai nodded. Yukio went back to teach is class. 

"Come on, Rin. Kasai pulled out the key and opened a nearby door. The room Yukio had mentioned lay on the other side. Rin's eyes widened. 

"Well don't just stand there!" Said a voice from inside the room.


	5. Vatican's Orders

Rin and Kasai stepped into the room. It looked like a disused classroom, but without any furniture. It was completely empty except for Shura. The only thing that made it seem like it could be a classroom was the chalkboard on the wall. Shura had a bag over her shoulder and a sword in her hand. Rin's hand went to his own sword eagerly, but Kasai slapped it away. "What did I tell you about starting at the end?"

"All right, Rin." Shura set the bag down and pulled out three candles. "Sit down." Kasai raised an eyebrow.

"Are you to help me train Rin?" Kasai asked.

"I'm here to make sure it all doesn't go to blazes, literally. If one or both of you goes crazy, I'm to kill you." Shura turned to Rin. "Light the two candles on the end, but not the one in the middle at the same time." Kasai looked thoughtful.

"A good challenge." Rin sat in front of the candles. They all erupted in flame. Shura and Kasai sighed. Shura handed Rin the bag of candles and told him to try again. Then she pulled Kasai aside.

"I had orders from the Vatican. I was to investigate a plot by Father Fujimoto and Mephisto Pheles. If the plot turned out to be Satan related, I was given the authority to eliminate it." Kasai bristled. "Relax. I'm not going to. But I may have to report you. Everyone saw you..wether they remember it or not is a different matter. It was pretty chaotic. And if they can prove you weren't here originally, they will go after Rin, and maybe Four-Eyed Chicken too."

"Four-Eyed Chicken..?" Kasai asked.

"Yukio. Anyway. Do you understand?" Kasai nodded. 

"Aw man!" Rin sat in a pile of melted candles. Kasai walked over to him, crouching down beside him.

"Think about scale. You are stronger than your flames. Only let out as much as you need. No more. A little flame, on the end of the two wicks. Focus." Kasai pulled out three more candles and lit the ones on the ends at the same time. Rin flopped on the ground. Kasai narrowed her eyes. "Are you giving up?" She asked, her voice deadly serious. Rin groaned. "I said are you giving up?" Kasai repeated, louder. Rin rolled over.

"No." Kasai smiled, placing another three candles in front of Rin. He focused, his eyes narrowing. Suddenly, all three wicks engulfed in flames.

"You still have work to do." Kasai said as Rin fell over again.


	6. Classes

Though Kasai technically didn't need classes, Rin did, and she couldn't train him while he was in class. So Kasai accompanied Rin to his classes. Technically Yukio and Shura had been entrusted with looking after Rin, but Kasai easily shared those duties as there was little risk of her losing control. The rest of Rin's classmates were confused by Kasai's presence, but in the chaos they were convinced by Yukio and Shura that they didn't know what they saw and it was certainly not Kasai erupting in Satan's blue flames and don't worry about it. Kasai had been enrolled in all of Rin's cram school classes. She did not have to attend the other high school.

"Here is all the material in Rin's classes so far." Yukio had given her a ton of work to do, but it's not like she had anything else. She had finished all the work before she had her first class. Rin sat up front with Shiemi, so she sat one table over.

"Has everyone done their homework?" The class mumbled a yes. Kasai was bored by the classes. She knew more about demons than any staff member here, save Sa- Mephisto. Kasai wasn't used to calling the King of Time 'Mephisto'. Back in the dorms, she easily finished the work for her classes in about ten minutes and went to find Rin. He was trying to sneak out when she got there.

"Hey, Rin." Kasai called as he raced out of his room. He turned just as Yukio came out of the door. 

"Come on, Rin! If you don't do the work you'll never become a Paladin!" Yukio beckoned Rin back inside. "Oh, Kasai. What did you need?"

"Well, I was going to ask Rin if he wanted to train, but he isn't done with any of his work yet?" Kasai shook her head. "If you don't do the work, not only will you never become a Paladin, but you'll never learn control and someone will kill you!" Rin slouched over to the two of them.

"Fine, fine!" Rin flopped over, papers in hand. 

"Here, let me show you." With Kasai and Yukio both helping Rin with his homework, they managed to finish in two hours. 

"That took forever!" Rin groaned, exhausted.

"We're done?" Yukio asked blearily. He was tired of having to wake Rin up every five minutes.

"Great!" Kasai leapt to her feet. "You know what this means, Rin?" Rin shook his head sleepily. 

"Do I want to know?" He mumbled.

"Training time!"


	7. Demon Alert!

Rin complained the entire walk to the classroom they used for training. "Come on! I just did work!" Kasai rolled her eyes, then stopped. "What?" Rin asked.

"Do you hear that?" Kasai tilted her head. Rushing water. "Look out!" Kasai shoved Rin into the nearest classroom as a wave of reddish water rushed past them. "Someone is here." Rin looked around wildly.

"What? Where?" Rin asked. Kasai shushed him, looking down the corridor, searching for someone. "Someone has accepted a demon, gaining it's power to defeat, well, me I guess. I feel so special." Kasai sighed.

"What? What do we do?" Rin asked.

"I'll deal with this." Kasai was about to move forward when she heard Rin gasp and a gun click. "Yukio. What do you want?"

"I can't let you loose on whoever it is. It could be a student." Yukio kept the gun pointed at Kasai as she turned around.

"Would you believe me if I told you I've never killed anyone and don't plan on it?" Kasai looked Yukio in the eye, staring at him.

"No." Yukio said simply, and took off down the hall, looking for the person.

"What the hell was that?" Rin gaped at his brother.

"Someone who doesn't understand trying to assert themselves with violence, the traditional human reaction to anything they don't understand." Kasai glanced over at the shocked Rin, obviously reeling from the rude awakening.

"I can't believe Yukio would.." Rin slumped on the wall.

Kasai's face hardened. "Well. He is an Exorcist." Kasai was a bit disappointed with Yukio for so blatantly showing his true colors in front of his brother. She knew what Yukio was getting at. He would never trust her, not matter if she was his sister. If he had to, Yukio would kill Rin. In that moment, Kasai made up her mind. She made the decision that she wouldn't let Yukio kill Rin. She would protect Rin from harm.

After all, they were siblings.


	8. The Red Water

"So what do we do now? Yukio asked us to wait." Rin asked.

"Yeah. Then wait." Kasai replied, as she started to follow Yukio. 

"Not if you're going."

Kasai sighed. "Fine." She took off after Yukio, to the source of the water. "It must be some sort of water demon, probably to counter our fire." Kasai smiled. "Like that'd work." Kasai thought about what Yukio had said. 'It could be a student.' Really? Could it? Huh. Maybe she underestimated the stupidity of the exorcists here. Demon-human contracts always work out poorly for the human. Demons are excellent liars, and even better at bending the truth. 

"So, how did this demon get in?" Rin asked.

"Someone must have let it in. Someone very weak, or very stupid got duped by this demon." Kasai answered. "The demon is this way!" Kasai didn't know if Yukio would go the right way. It was probably better if he didn't. 

"How do you know?" Rin asked. 

"Can't you feel it?" Kasai responded. "That room!" Kasai burst through the door. It wasn't a student. It was definitely a teacher, though one Kasai hadn't met before. Rin didn't seem to know him either. A ghostly form was behind the man, composed entirely of the red water. The man was mumbling. Yukio hadn't made it there yet.

"Letting Satan's kids roam through the school unchecked! Unbelievable!" The teacher muttered. "Bound to be a disaster!" 

"Yes, a disaster. You can do something about it, if you agree to my power." The demon leered. 

"SHUT UP!" Kasai yelled. Calling Satan's flames, she blasted the demon, making sure not to harm the teacher.

"Ack!" The demon shrugged off the attack. "You're going to have to do better than that."

"Is that a challenge?" Kasai answered, in a deathly cold voice.

"Yes." The demon hissed back. Kasai put her arms in an X in front of her, eyes blazing with Satan's fire.

"You! Demon of Gehenna! Obey the heir to the Throne of Gehenna!" Kasai yelled. The demon struggled, but it was over taken. "I banish you!" Flames swirled through the air, and the demon was gone. "Wow, that was a powerful one." Kasai said before blacking out.


	9. Yukio's Anger

Apparently, just after she passed out, Yukio had made it to the classroom. When she woke up, Rin was sitting next to her, head down. "I take it Yukio didn't like our little excursion?" Kasai said, jokingly. Rin shook his head.

"No, he was mad. Really mad. He said it was okay this time, because no one got hurt and the teacher didn't remember anything, but he also said he'd be watching you. And me. But mostly you." Rin informed her.

Kasai roller her eyes. "I'll be fine." She stretched, standing up. "The bigger problem is more the exorcists here. They're losing faith in Mephisto. And when that happens, nothing will stop them from calling the Vatican and killing us both."

"Got that right." Shura was standing in the doorway. "So you both are coming on a mission. You both are making it up to the school by taking out a demon, with the supervision of the rest of your class of course. No flames."

"Got it." Kasai said. "But does Rin?" 

Rin shrugged. "Sure."

"I can pretend to be a knight/tamer. Rin, you're being a knight, right? You need a different sword." Kasai looked at Shura.

"On it." Shura tossed them both a wooden sword. "What did you mean by tamer?"

"Well, Satan is the lord almighty in Gehenna. He taught me a trick or two. Lower level demons I can bend to my will easily. The higher the level, the harder." Kasai explained. 

"Interesting." Shura said. "Well, come with me." She started walking. Rin and Kasai hurried after. Kasai held her sword loosely in her hand, toying with her uniform tie. They met up with the rest of the students shortly. 

"Hey, Rin! How are you?" Sheimi asked cheerfully. 

"Oh, I'm fine." Rin said.

"So what's this mission?" Kasai asked Shura, tossing the wooden sword back and forth in her hands. She was dying to add a touch of flames to it. It would become much deadlier weapon. But Shura had been clear, and Kasai didn't want to deal with the Vatican right now. 

"Just taking out a mid level demon in a nearby forest. Should be easy." Shura said. She turned to the students. "All right everybody, suit up! We've got some hands on experience to gather!"


	10. Mid-Level Demon

Kasai yawned. They had been walking for a while. Contrary to Shura's exciting buildup, the trip to this forest was long, and very, very boring. "Are we there yet?" Rin asked, for the 5749376th time. Shura just shook her head, no longer dignifying the question with a verbal response. Rin gave an exaggerated sigh, but he wasn't the only one aching for some action. 

"This walk is taking forever!" Bon complained. "I thought you said this forest was nearby!" Shura sighed. The students had been constantly pestering her for answers about the demon, the forest, the mission, anything, really. 

"It is nearby. But since we're all carrying equipment, it's going to take a little while!" Shura said. She had already answered that question, actually, several times. But the students weren't satisfied. 

"Are we going to be there soon?" Shiemi asked timidly. 

"Are we there yet?" Rin asked, for the 5749377th time. Shura sighed, shaking her head again. 

"Shut up! We aren't there yet, and asking isn't going to get us there any faster!" Kasai said, punching Rin in the arm. She was bored, but she was trying to keep her mind sharp by doing some sword fighting practiced. She lunged at Rin, moving in for a strike. He quickly blocked. 

"Hey! What was that for?" Rin asked. Kasai smirked, leaping back and sheathing her sword. She had half expected him not to block it, in this bored state that the class was in. 

"To keep your mind sharp." Shura said, before Kasai could answer. She was grinning. "Good thinking, Kasai. I think I like having you on this team." Kasai returned the grin. 

"But seriously, are we there yet?" Rin asked again. 

"Yes." Shura said, stopping. Most of the class didn't register that at first, and kept walking. Kasai came to a halt next to Shura, and watched several students walk straight into trees. 

"Oh! We're here!" Rin said, jumping up and down. "I can't believe it! We finally made it! We're gonna fight some demons!" Rin cheered. He danced around, and Kasai laughed. 

"We still have to set up camp. It's not quite nighttime yet, and the demon won't come out until then, not at this level." Shura said, beginning to give tasks to people. They walked until they found an appropriate clearing, where the students set to work preparing the camp for the demons arrival. Everything was going well. Kasai didn't feel like that though. She kept glancing over her shoulder. A mid level demon, right? Then why was she feeling so uneasy? 

Why did she feel like there was something else here?


	11. Camp is Boring Too

"So we walked all this way." Rin said. Kasai nodded. "A long, boring walk." Kasai nodded again. "Just so we could sit here, lounging around, doing nothing, until nighttime?" Rin raged. 

Kasai had to agree with Rin. The walk had been long and arduous.

"This is how it works, kid." Shura said, grinning. "You can spar with Kasai again." Rin shook his head violently before lapsing into silence. 

Kasai turned her head to see how the others were fairing. Many where eating dinner. She glanced up at the sky. It had gotten considerably darker. Kasai stood. She could feel her power just within her reach, lying dormant. She lifted her wooden sword, resting it on her shoulder. 

"Is it almost time?" Rin asked, grinning. He reached for his sword, holding it out in front of him. 

"It is." Shura replied. "Everyone, finish your dinner and gather over here." People began shuffling about, hurrying to down their dinners and rush over to Shura. Once everyone was done, they all stood in a semi-circle around Shura. 

Kasai smirked slightly, the corner of her lips tugging upwards. She could feel like energy and tension, and she could sense the demons out there, just waiting for Kasai to get them. 

Shura put some more protective measures on the students, dousing them with holy water and such. She almost got Rin and Kasai before quickly stopping. "That could have been bad." She chuckled, putting the holy water down. 

"So, what's our goal?" Bon asked. Shura nodded, turning to face them. 

"There is a mid-level demon in this forest." Shura said. "Should be easy enough. Split into groups of two and I'll send you each off in a direction." Shura turned to Kasai as students rushed to find their friends. "You, Rin." She commanded. 

"Looks like we're together." She said, glancing at Rin. Sheimi ended up in their group as well. Shura spread them out evenly. 

"Everyone, stay safe." Shura said. "I'll be watching over everything." She glanced at Rin, and Kasai. 

"Go!"


End file.
